I've always had very good results exporting to youtube directly from FCPX. But youtube doesn't always show the full quality depending on the internet connection, at times it defaults to a lower quality. Check the settings in the viewer.

For me,the Vimeo/YT presets (with a few minor changes) in FCPX and Compressor have worked quite well. As an experiment, I've tried uploading a Pro Res file; IMO, it wasn't worth the extra upload time…at least for that one test.